The movement of objects through the influence of electromagnetic fields is a widespread engineering practice with many advantages over mechanical alternatives. One application of this practice is the electromagnetic levitation of metals for crucibleless processing. In this application a body of metal is suspended in space by the induced eddy current repulsion between the metal and a suitably shaped alternating magnetic field. Melting is induced by making the eddy currents intense enough or through the application of an additional RF field. The molten metal is then processed and separated while it is suspended in space, thus never coming in contact with a crucible. Very pure, uncontaminated metal products are obtained this way.
A specific electromagnetic levitation melt system designed to combine the melting, melt treatment, and pouring procedures into a single operation was developed at the University of Alabama and described in Levitation-Melting Method Intrigues Investment Casters (March 1991) Advanced Materials and Processes, 42-45. As shown schematically in FIG. 1 a metal 101, which is to be processed, initially rests on top of a base plate 102 which has a hole 103 in its center, the hole's diameter being slightly smaller than that of the metal billet. When power is supplied to a set of induction coils 104, a current is inducted in metal 101 causing it to begin to heat up and gradually melt, melting from top to bottom. The electromagnetic force field created by the interaction of the induced current and its associated magnetic field has a rotational component which stirs the melt. The irrotational component of the field pushes against the outside surface of the melt. When the center of the bottom of the billet melts, the liquid metal drops through hole 103 into a mold 105.
A second type of related application is electromagnetic pumping, where a conducting fluid is propelled along a channel through the interaction of induced currents and static or alternating magnetic fields. An overview of such propulsion systems is given by D. L. Mitchell et al. in an article entitled Induction-Drive Magnetohydrodynamic (MHD) Propulsion in Journal of Superconductivity, 6 (4) (1993) 227-235. The authors describe the early research in applying MHD propulsion systems to seagoing vessels during the 1960's through the current research using high field superconducting magnet technology. A cited example of recent research in this area is the Yamato I, a 280-ton test vessel utilizing two MHD thrusters incorporating Ni:Ti superconducting magnets cooled by liquid-helium cryostats. The electrical conversion efficiency for the Yamato I thrusters is only a few percent. The authors state that increasing the efficiency would require, either singly or in combination, an increase in the magnetic field strength, the size of the propulsion units, or the conductivity of the seawater. The authors also discuss the use of electromagnetic propulsion systems for pumping hazardous materials and for controlling the liquid sodium flow in breeder reactors.
A third type of application is known as Maglev. Entire transport vehicles (e.g., trains) can be suspended over guiding rails to yield a nearly frictionless high speed mode of transport. A fourth class of applications involve the sudden exchange of energy from an electromagnetic form to a kinetic form or vice versa. The former is the foundation of rail-gun kinetic energy weapons. The latter is the preferred approach for the production of MegaGauss fields in small regions through explosive flux compression.
The most general force law at work in the above applications is the Lorentz force between a current and a magnetic field: F=.intg.I.times.B dl. The efficiency of such a force for accomplishing the propulsion of matter is then in general proportional to the square of the magnetic field. This is clear when the current I is induced by the magnetic field B itself. Since the power wasted is proportional to the Joule heating of the conducting material, even when the current is supplied by a separate source it is more advantageous to have a high B-field, low current system than a low B-field, high current system. Then for a constant force F, since the power lost goes as I.sup.2 R=[F/(Bl)].sup.2 R, the advantage also goes as the square of the magnetic field. For this reason it is desirable to generate the strongest magnetic fields possible.
At present, the most efficient magnetic field generation systems utilize superconductors capable of sustaining thousands of Amperes with negligible loss. Their main disadvantages are the requirement for cryogenic cooling and the eventual limitation that high field strengths place on the superconducting state. The alternative of using conventional conductors is viewed as impractical because the high currents required to produce a strong magnetic field in a given region of space would eventually melt the conductors.
From the foregoing, it is apparent that a method by which the magnetic field produced by an electric current can be multiplied in amplitude to the desired strength so that high field strengths can be produced by current carrying conductors with minimized joule heating of the conductors is desired.